1. Field of the Invention
The present invention relates to a developing device and an image forming apparatus.
2. Description of the Related Art
Recently, a demand for high-speed output of a color image is increased in a color printer using a one-component developer. For example, a high-speed output is also demanded in a color image forming apparatus in which a plurality of color image forming units are disposed in parallel for forming a color image.
FIG. 1 is a schematic sectional view of one exemplary color image forming apparatus, which is a color printer. This image forming apparatus includes latent electrostatic image bearing members (901Y, 901M, 901C and 901K) on which a latent electrostatic image is formed, and a developing device 902 which supplies a toner to the latent electrostatic image bearing member. The developing device 902 includes a developing roller 921 which is made of an elastic body and configured to abut against the latent electrostatic image bearing member, followed by supplying a small-particle diameter toner with an average particle diameter of 6 μm or less to the latent electrostatic image which has been formed on the latent electrostatic image bearing member to thereby visualize the latent electrostatic image; a toner supplying roller 922 which is configured to supply the toner to the developing roller 921; and a toner layer-regulating member 923 which is configured to abut against the developing roller 921 to thereby thin down a toner layer on the developing roller 921. The image forming apparatus further includes a cleaning device 904 which is configured to remove the toner remaining on the latent electrostatic image bearing member after a toner image is transferred to an intermediate transfer belt 903.
Once the color image forming apparatus shown in FIG. 1 is activated and starts printing, the latent electrostatic image bearing members (901Y, 901M, 901C and 901K) are rotated. Firstly, a charging device uniformly charges surfaces of the latent electrostatic image bearing members (901Y, 901M, 901C and 901K). An exposing device having a write optical system then irradiates an optical image to thereby form a latent electrostatic image on the latent electrostatic image bearing members (901Y, 901M, 901C and 901K). The developing device 902 supplies the toner to the latent electrostatic image to thereby form the toner image. Then, the toner image is conveyed to a primary transfer nip portion between the latent electrostatic image bearing members (901Y, 901M, 901C and 901K) and an intermediate transfer belt 903, where the toner image is transferred to the intermediate transfer belt 903. The toner image is transferred from each of the latent electrostatic image bearing members for yellow (Y), cyan (C), magenta (M), and black (B) to the intermediate transfer belt 903, resulting in forming a four-color toner image. The four-color toner image is conveyed to a secondary transfer nip portion in which a secondary transfer roller 905 is provided, and transferred to transfer paper P. The four-color toner image on the transfer paper P is heated and pressured by a fixing device 906 to thereby be fixed on the transfer paper P.
FIG. 2 illustrates the developing device 902 shown in FIG. 1 in detail. The developing device 902 contains a predetermined amount of a non-magnetic one-component toner. This toner is supplied to the toner supplying roller 922 which is disposed below the developing device 902 and which is made of foam. The toner supplying roller 922 and the developing roller 921 are rotated at a predetermined linear velocity ratio in a direction indicated by an arrow in FIG. 2 to thereby supply the toner from the toner supplying roller 922 to the developing roller 921. The developing roller 921 is generally made of an elastic member (e.g., rubber) or metal having a blasted surface. A surface of the developing roller is roughened in order to ensure conveyance of the toner.
The toner adhered on the developing roller 921 is negatively charged through friction at a blade nip portion between the toner layer-regulating member 923 and the developing roller 921. Notably, as the toner layer-regulating member 923, so-called doctor blade is typically used. Specifically, a folded portion is formed by folding a metal plate and allowed to abut against the developing roller 921. A needed amount of toner layer is formed on the developing roller 921 at downstream of the blade nip portion in a rotation direction. The developing roller 921 comes into contact with the latent electrostatic image bearing member 901, and rotates at a predetermined linear velocity ratio η to a linear velocity of the latent electrostatic image bearing member 901 (linear velocity of the developing roller/linear velocity of the latent electrostatic image bearing member), to thereby supply the toner to the latent electrostatic image bearing member 901. Thus, a toner image is formed on the latent electrostatic image bearing member 901.
Recently, a demand for high-image quality is increased also in a color image forming apparatus using a one-component developing device which is small-sized and suitable for lowering cost. Accordingly, the one-component developing device is also required to increase image quality by means of a small particle diameter toner.
However, the small particle diameter toner is easily aggregated under a stress. As a result, aggregates of the toner are produced at a blade nip portion between a regulating blade (toner layer-regulating member) and a developing roller, whereby the toner stays at the blade nip portion. Then, the toner sticks firmly to the regulating blade, which is problematic.
In order to solve the above problem, there has been proposed a developing device including a toner bearing member which bears a toner on its surface, which develops with the toner a latent electrostatic image on a latent electrostatic image bearing member, and which has convex portions regularly arranged on the surface, wherein a ten-point average roughness of the convex portions is smaller than a value obtained by subtracting a value three times as large as standard deviation in a particle size distribution of the toner from a volume average particle diameter of the toner; and an image forming apparatus including the developing device (see Japanese Patent Application Laid-Open (JP-A) No. 2008-299015). In this proposed technology, the problem in which the toner sticks firmly to the regulating blade has been solved by using the toner bearing member which has regularly arranged convex portions on its surface.
However, in this proposed technology, a desired developability cannot be achieved in spite of controlling an electric charge amount of the toner, that is, developing efficiency is unsatisfactory, which is problematic.
Thus, at present, keen demand has arisen for a developing device which can achieve satisfactory developing efficiency.